Nonaqueous-electrolyte batteries represented by lithium secondary batteries are extensively used as power sources for small portable terminals, mobile communication apparatus, and the like because these batteries have a high operating voltage and a high energy density. The positive active materials for lithium secondary batteries are required to stably retain their crystal structure even when subjected to repetitions of lithium insertion/extraction and to have a large reversible electrochemical capacity.
Presently, an Li—Co composite oxide having an α-NaFeO2 structure (hereinafter the composite oxide is referred to as LiCoO2) is mainly used as a positive active material for lithium secondary batteries. LiCoO2 is capable of stable insertion/extraction of lithium ions at an operating potential as high as 4 V and stably retains its crystal structure even in repetitions of lithium ion insertion/extraction. LiCoO2 hence shows a high energy density and simultaneously attains high charge/discharge cycle performance.
However, since cobalt, which is a constituent element for LiCoO2, is a scare element and expensive, many investigations have been made on the use of an Li—Ni composite oxide (hereinafter referred to as LiNiO2) as a substitute for LiCoO2. LiNiO2 has an α-NaFeO2 structure like LiCoO2 and has almost the same operating potential width as LiCoO2. LiNiO2 is hence expected to attain high electrochemical performance. In non-patent document 1, the relationship between lithium extraction amount and crystal lattice is investigated by X-ray powder diffractometry. It has been reported therein that even when lithium is extracted from a positive electrode in charge/discharge at a depth of up to 200 mAh/g, the layer-to-layer spacing is stably maintained. However, there has been a problem that repetitions of charge/discharge at such a depth result in an abrupt decrease in discharge capacity.
Techniques for eliminating that problem are being extensively investigated which comprise displacing part of the nickel sites in an LiNiO2 structure by an element of a different kind. For example, patent document 1 discloses a technique in which part of the nickel sites are displaced by an element such as cobalt or aluminum in an amount of substantially about 20% at the most to thereby improve charge/discharge performance and thermal stability. However, the composite oxide obtained by this technique is still insufficient in charge/discharge cycle performance and thermal stability as compared with LiCoO2, although slight improvements in charge/discharge cycle performance and thermal stability are attained.
Furthermore, many investigations have been made on the use of an Li—Mn—Ni composite oxide obtained by displacing part of the nickel sites by manganese and an Li—Mn—Ni—Co composite oxide obtained by displacing part of the nickel sites by manganese and cobalt.
Techniques concerning the Li—Mn—Ni composite oxide are reported, for example, in patent documents 2 to 4. However, investigations made by the present inventors revealed that use of any of these techniques poses a problem that not only the electrochemical capacity at an operating voltage of around 4 V is considerably lower than that attained with LiNiO2, but also charge/discharge cycle performance and high-rate discharge performance also are insufficient.
With respect to techniques concerning the Li—Mn—Ni—Co composite oxide, there is a report in, e.g., patent documents 5 to 12 that the composite oxide shows a higher energy density than the Li—Mn—Ni composite oxide. The Li—Mn—Ni—Co composite oxides described in these reports have a composition in which the proportions of cobalt and manganese in the 6b sites each are low and nickel is the main component. The term 6b sites as used herein means a Wyckoff position.
On the other hand, a technique for producing an Li—Mn—Ni—Co composite oxide using a precursor in which the proportions of manganese and cobalt in the 6b sites are high and the two components have been mixed with each other extremely evenly was recently reported in non-patent document 2 and non-patent document 3. There is a statement therein to the effect that in the LiCo1/3Ni1/3Mn1/3O2 reported therein, the manganese, nickel, and cobalt occupying the 6b sites are regularly arranged to thereby form a superlattice and this brings about a stable crystal structure. Because of this, lithium can be extracted without causing a phase change even at high voltages. It has also been found that this composite oxide further has excellent thermal stability probably because oxygen repulsion between c-axes after lithium elimination is relieved. It has further been found that although manganese is contained in the crystal lattice, lithium secondary batteries produced with this composite oxide are almost free from the phenomenon in which manganese dissolves in the electrolyte as in the case of using a spinel manganese material. The nonoccurrence of the phenomenon is thought to imply that the manganese ligand field is free from a Jahn-Teller distortion because the manganese has a valence of 4. Consequently, the composite oxide is advantageous in that adverse influences on battery performances, including the problem that the negative electrode comes to have increased resistance due to the manganese dissolution phenomenon, are diminished.
However, investigations made by the present inventors revealed that the problem of the decrease in discharge capacity with many repetitions of charge/discharge cycling still remains unsolved even with any of those techniques. There has been a desire for a technique for further improving charge/discharge cycle performance.
(Non-Patent Document 1)
T. Ohzuku, A. Ueda, and M. Nagayama, J. Electrochem. Soc., (U.S.A.), 1993, Vol.140, No.7, pp.1862-1870
(Non-Patent Document 2)
Y. Koyama, I. Tanaka. H. Adachi, Y. Makimura, N. Yabuuchi, and T. Ohzuku, Dai 42-kai Denchi Tôronkai Yokô-shû, (Japan), 2001, pp.50-51
(Non-Patent Document 3)
Y. Makimura, N. Yabuuchi, and T. Ohzuku, and Y. Koyama, Dai 42-kai Denchi Tôronkai Yokô-shû, (Japan), 2001, pp.52-53
(Non-Patent Document 4)
C. S. Jhonson, S. D. Korte, J. T. Vaughey, M. M. Thacherey, T. E. Vofinger, Y. Shao-Horn, and S. A. Hackney, J. Power Sources, (Holland), 1999, Vol.81-82, pp.491-495
(Non-Patent Document 5)
K. Numata, C. Sasaki, and S. Yamanaka, Chemistry Letters, (Japan), 1997, pp.725-726
(Patent Document 1)
JP-A-9-231973
(Patent Document 2)
Japanese Patent No. 3,008,793
(Patent Document 3)
Japanese Patent No. 3,047,693
(Patent Document 4)
Japanese Patent No. 3,064,655
(Patent Document 5)
JP-A-2000-260480
(Patent Document 6)
JP-A-2000-260479
(Patent Document 7)
JP-A-2000-268878
(Patent Document 8)
JP-A-2000-353525
(Patent Document 9)
JP-A-10-255846
(Patent Document 10)
JP-A-8-37007
(Patent Document 11)
JP-A-2000-58068
(Patent Document 12)
JP-A-2000-277151
(Patent Document 13)
JP-A-11-317224
(Patent Document 14)
JP-A-2000-3706
(Patent Document 15)
JP-A-11-312519
(Patent Document 16)
JP-A-11-307093
The invention has been achieved in view of the problems described above. An object thereof is to provide a positive active material for lithium secondary batteries which is capable of giving a lithium secondary battery having a high energy density and excellent charge/discharge cycle performance. Another object thereof is to provide a lithium secondary battery having a high energy density and excellent charge/discharge cycle performance.